Romero-Sarmiento et al
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5. Discussion 376
5.1. Thermal maturity 377
According to LECO results, the TOC values for these Scottish 378
samples are typical for coals (60.8 and 71.9%; Table 1). Based on such 379
values of TOC, the average T max
(426 °C) from Rock-Eval analyses, as 380
well as the vitrinite reflectance (0.45 %), the rank of the studied coals is 381
of subbituminous A, i.e. an immature stage of thermal evolution (Fig. 3). 382
This interpretation is supported by molecular thermal maturity 383
indicators from the aliphatic and aromatic fractions. First, the CPI is 384
high (>1) due to odd predominance (Table 1; Bray and Evans, 1961). 385
Second, the thermally unstable C 29 ββ hopane is observed in all samples 386 Romero-Sarmiento et al., 20 (Fig. 8; Seifert and Moldowan, 1980). Third, the high thermal maturity 387 markers in sediments 18α-22,29,30-trisnorneohopane (Ts) and αββ- 388 steranes (Fig. 9) are absent or very low in abundance (Seifert and 389 Moldowan, 1978, 1986). This is further attested by the aromatic 390 hydrocarbon distribution of MN, EN, DMN, TMN, TeMN and MP isomers 391 again indicating that the coals are immature (Table 6; Radke et al., 392 1982, 1986; Alexander et al., 1985; van Aarssen et al., 1999). For 393 instance, the TeMN compounds show the predominance of 1,2,5,6- and 394 1,2,3,5-TeMN over the 1,3,6,7-TeMN (Fig. 16; van Aarssen et al., 1999). 395 The distribution of methyl- and ethyl-naphthalene homologues with 2- 396 MN and 2-EN dominating is typical for immature samples (Fig. 14; 397 Table 6; Radke et al, 1982). The alkylphenanthrenes 9-MP and 1-MP are 398 highly abundant, with 9-MP being predominant (Fig. 17) is another 399 feature of immature samples (Radke et al., 1982). It is interesting to 400 note, however, that depending on the maturity index either WS or SKT 401 samples may be considered to be the more mature. WS samples are 402 more mature with DNR, TNR1 and TNR2 while SKT samples are more 403 mature with TMNr, TeMNr, MPI 1 and MPI 2 (Table 6). This feature 404 probably reflects the control exerted by the source of the organic matter 405 and depositional environment on these compounds at low maturity 406 degree.
407
408 5.2. Biomarker distributions and palaeoenvironmental conditions 409
Romero-Sarmiento et al., 21 Like most coals, the investigated ones are typically terrestrial. This 410 is evidenced by petrographic examination (Table 2, Fig. 4), palynology 411 (Spinner, 1969; Spinner and Clayton, 1973), and biomarker 412 distributions. In addition to the typical odd numbered long-chain n- 413 alkanes (Eglinton and Hamilton, 1967), several aliphatic and aromatic 414 compounds generally used as terrestrial biomarkers have been detected 415 in our Carboniferous coals (Table 7). 416 The Carboniferous sedimentary rocks in the Midland Valley of 417 Scotland consist of cyclical sequences of coals, oil shales, limestones, 418 shales, mudstones, siltstones and sandstones (Murchison and 419 Raymond, 1989; George, 1992). Coals are usually deposited in swampy 420 terrestrial environments such as deltas and regions with poor drainage. 421 Accordingly, a variety of depositional environments, all relatively 422 shallow-water and predominantly deltaic, has been assigned to these 423 Carboniferous rocks (Murchison and Raymond, 1989). In the case of a 424 deltaic environment (such as in our case) marine intrusions may occur 425 which may be evidenced by means of palynomorphs and lipid 426 biomarkers. Since organic particles of marine origin, i.e. alginite and 427 marine palynomorphs, and marine biomarkers (e.g. cheilantanes) are 428 absent, the (low proportion of) short chain n-alkanes in our coals (Fig. 429 6) must be considered to be of bacterial rather than marine origin. 430 Other arguments for a purely terrestrial depositional environment 431 are the high Pr/Ph (Table 1) and Pr/n-C 17 vs. Ph/n-C 18 ratios (Fig. 7; 432 Hunt, 1995). These ratios characterize the Scottish coals as Type III OM 433
Romero-Sarmiento et al., 22 which is usually derived from terrestrial plants (Peters and Moldowan, 434 1993) deposited under oxidizing conditions, and in particular, the WS 435 coal level (Fig. 7). 436 A bacterial contribution to the OM is further evidenced by the 437 occurrence of branched alkanes (Fig. 6; e.g. Shiea et al., 1990), 438 hopanoids (Figs. 5 and 8; Ourisson et al., 1979) and bicyclic alkanes of 439 the drimane and homodrimane series (Fig. 11; Noble, 1986; Noble et al., 440 1987). Aromatic hydrocarbons often associated to a microbial source 441 include 1,2,3,5,6-pentamethylnaphthalene (1,2,3,5,6-PMN; Fig. 13; 442 Table 5; Bastow et al., 1998) and 1,3,6,7-tetramethylnaphthalene 443 (1,3,6,7-TeMN; Fig.16; e.g. Jiang et al., 1998). 444 Steroids have also been used to differentiate depositional settings 445 (Peters et al., 2005). Huang and Meinschein (1979) proposed a useful 446 ternary diagram to identify the source of the OM which can be 447 constructed by the distribution of C 27 , C
28 and C
29 sterols (Fig. 10). 448 Based on their results, the dominant source of C 27 sterols is 449 zooplankton, C 28 sterols are generally components of phytoplankton and 450 C
sterols are mainly derived from terrestrial plants. There are many 451
exceptions on these rules; for instance, C 29 sterols are also found in 452 marine diatoms and dinoflagellates (e.g. Rampen et al., 2010). 453 Nevertheless, the method seemed reliable for other Palaeozoic (Middle 454 Devonian) coals which have a high predominance of C 29 steranes (e.g. 455 Fowler et al., 1991; Kashirtsev et al., 2010). Considering this, the 456 dominance of C 29 relative to C 27 steranes in these coals further 457 Romero-Sarmiento et al., 23 evidences their terrestrial origin (Table 1; Fig. 9). Consistent with the 458 shallow-water environment proposed for Scottish coals (Murchison and 459 Raymond, 1989), the steroids plot in a transitional estuarine-bay 460 environment (Fig. 10). It is interesting to note that short chain C 19 - C
20
461 steroids are abundant in SKT coals (Fig. 9). Although these compounds 462
are not commonly described from coals, they have been reported in a 463
Devonian liptobiolith (Kashirtsev et al., 2010). The relative abundance of 464
these C 19 - C 20 steroids may be characteristic for some primitive coals, 465 suggesting that they could be more specific biomarkers of early plants. 466 The occurrence of the combustion-derived PAHs pyrene, 467 fluoranthene, benzo[a]anthracene, chrysene and triphenylene (Fig. 13) 468 can be related to the significant presence of the fusinite group, most 469 particularly pyrofusinite, in the organic matter. Charred or fusinized 470 plant debris have been frequently reported in coals from the Midland 471 Valley of Scotland (Murchison and Raymond, 1989; Scott and Jones, 472 1994; Falcon-Lang, 2000) and combustion-derived PAHs have been 473 observed in coal extracts (Murchison and Raymond, 1989). These 474 compounds and macerals testify that fire events took place 475 contemporarily with coal deposition. Fire events in coals are often 476 related to lightning, but in the case of the Midland Valley of Scotland, 477 volcanic activity also appears as a major cause of wildfires (Murchison 478 and Raymond, 1989; Scott and Jones, 1994; Falcon-Lang, 2000). 479
5.3. Linking biomarkers, palynology and palaeobotany 481
Romero-Sarmiento et al., 24 On the basis of the diverse palynomorph assemblages described by 482 Spinner (1969) and Spinner and Clayton (1973), a relatively detailed 483 reconstruction of the flora giving rise to the SKT and WS coals can be 484 provided, though more details are available for SKT coals than for WS 485 coals. In WS coals, megaspores are dominated by Zonalesporites and/or 486 Setosisporites, followed by Lagenicula (Spinner, 1969). Cystosporites are 487
also present in minor proportion. In SKT coal, dominant megaspores are 488
Lagenicula and Setosisporites followed by ?Bacutriletes and 489
Zonalesporites (Spinner and Clayton, 1973). Miospores assemblages are 490
generally dominated by Lycospora, Calamospora and Densosporites 491
(Spinner and Clayton, 1973). Cingulizonates miospores should also be 492
abundant in SKT coal. The megaspore Zonalesporites as well as the 493
miospores Densosporites and Cingulizonates were mostly produced by 494
Bodeodendron/Sporangiostrobus (recently identified as equivalent to 495
Omphalophloios) which was a sub-arborescent lycopsid belonging to the 496
Isoetales group (Wagner, 1989; Taylor et al., 2009; Opluštil et al., 2010). 497
Bacutriletes miospores were produced by herbaceous and sub- 498
arborescent lycopsids among which Bodeodendron/Sporangiostrobus 499
(Eble, 1996). Setosisporites megaspores mainly derived from 500
Bothrodendron (e.g. Phillips, 1979). Lagenicula megaspores and some 501
Lycospora miospores were produced by Paralycopodites (also known as 502
Anabathra; DiMichele and Phillips, 1994). Finally, Cystosporites 503
megaspores and some Lycospora miospores were produced by 504
Lepidodendron (DiMichele and Phillips, 1994). Bothrodendron, 505
Romero-Sarmiento et al., 25
506 belonged to the Lepidodendrales group (Taylor et al., 2009). It must be 507 noted that these different plants are mostly documented from the Late 508 Carboniferous and afterwards, though the corresponding spores 509 appeared earlier, sometimes in the Devonian (Glasspool et al., 2000; 510 Opluštil et al., 2010). The plants from which these spores derived in 511 Viséan coals are not firmly identified, but it is highly probable that they 512 correspond to several groups or families of lycopsids (Glasspool et al., 513 2000), in particular to Isoetales and Lepidodendrales which already 514 existed during the Viséan (Taylor et al., 2009). Accordingly, though the 515 same lycopsids were present in both coal levels, WS coals vegetation is 516 marked by a higher contribution of Isoetales (Bodeodendron-like) while 517 Lepidodendrales, and in particular Paralycopodites-like plants, were 518 more abundant in the vegetation of SKT coals. In addition to these 519 dominant plants, the miospore assemblages (Spinner and Clayton, 520 1973) document the presence of a notable contribution of Calamites 521 sphenopsids and of small ferns, in particular Botryopteridaceae and 522 Zygopteridaceae. Marattialean tree ferns and the arborescent lycopsid 523 Sigillariaceae are also indicated, as well as Lyginopteridacean 524 pteridosperms documented by the presence of the spore Schulzospora 525 (Spinner and Clayton, 1973). Lycopsids were abundant spore producers 526 (DiMichele and Phillips, 1994) and the abundance of spores observed in 527 the organic matter (Table 2; Fig. 4) supports a significant contribution 528 of lycopsids to our Scottish coals. By its diversity and dominance of 529 Romero-Sarmiento et al., 26 lycopsids, this floral assemblage appears relatively comparable to the 530 anatomically preserved floras of Glenarbuck and Pettycur in Scotland 531 (Scott et al., 1984), which are of Viséan age but slightly older than our 532 studied coals. It is notable that although there are some Viséan records 533 of Cordaites (e.g. Wang, 1998), Cordaites remains were not reported 534 from Glenarbuck and Pettycur (Scott et al., 1984) nor Cordaite spores 535 (e.g. Florinites) in the palynofacies from our Scottish coals (Spinner, 536 1969; Spinner and Clayton, 1973). 537 The relatively similar lycopsid-dominated floras observed in SKT 538 and WS coals (Spinner, 1969; Spinner and Clayton, 1973) are 539 consistent with the relatively similar biomarker content observed in 540 both coals. Some aromatic biomarkers detected in this study can be 541 related to specific plant taxa (Table 7). For instance, the presence of 542 alkyldibenzofurans (MDBFs) indicates lichen input (Fig. 13; Radke et 543 al., 2000). Although lichen fossils or specific lichen spores have not 544 been reported in Scottish coals (Spinner, 1969; Spinner and Clayton, 545 1973), it can be safely assumed that Euramerican Coal Measure forests 546 were a good potential habitat for lichens. Among the less specific 547 biomarkers, the detected combustion-derived PAHs cannot be 548 associated to specific kind of plants (Oros and Simoneit 2000a,b; Oros 549 et al. 2006; Iinuma et al., 2007). It is also the case for 1,6- 550 dimethylnaphthalene (1,6-DMN; Fig. 14), 1,2,5-trimethylnaphthalene 551 (1,2,5-TMN; Fig. 15), pimanthrene (1,7-DMP; Fig. 16) and cadalene, 552 which can be produced by aromatization of several different terpenoid 553
Romero-Sarmiento et al., 27 structures and/or which precursors are widespread among terrestrial 554 plants (Table 7). Ionene is a degradation product of β-carotene (Day and 555 Erdman, 1963). It is often observed in coal extracts (e.g. Wang and 556 Simoneit, 1990), but also can be produced from thermal degradation of 557 marine and lacustrine sediments (Day and Erdman, 1963; Achari et al., 558 1973). 4β(H)-eudesmane is another non-specific land plant biomarker. 559 This compound has been rarely observed in Palaeozoic coals (del Río et 560 al., 1994; Dzou et al., 1995) and is considered to derive from evolved 561 land plants such as angiosperms and gymnosperms (e.g. conifers; 562 Noble, 1986). However, bicyclic alkanes with an eudesmane skeleton 563 have been also identified in recent bryophytes (Asakawa, 2004). Its 564 oldest reported occurrence in sediments, to our knowledge, is Middle 565 Pennsylvanian (Dzou et al., 1995). 566 The diterpenoids ent-beyerane, abietane, ent-kaurane and 567 phyllocladane have been previously described in Carboniferous (e.g. 568 Schulze and Michaelis, 1990; Fleck et al., 2001; among others) and 569 Permian coals (e.g. Noble, 1986; Noble et al., 1985). Abietane precursors 570 occur in all conifer families (Otto and Wilde, 2001; Cox et al., 2007) and 571 this compound has been recently described in the pyrolysate of Late 572 Carboniferous amber (Bray and Anderson, 2009). Kaurane skeletons 573 have been observed in different kinds of plants, and in particular 574 bryophytes (e.g. Noble, 1986; Chopra and Kumra, 1988; Asakawa, 575 2004). The occurrence of this component in very early terrestrial OM 576 (e.g. Sheng et al., 1992; Disnar and Harouna, 1994; Kashirtsev et al, 577
Romero-Sarmiento et al., 28 2010; Romero Sarmiento et al., 2010) can be therefore related to 578 bryophytes, which represent the earliest land plants. Phyllocladane and 579
580
Pinaceae (e.g. Noble, 1986; Schulze and Michaelis, 1990; among 581
others). However, the oldest recorded occurrences of phyllocladane and 582
ent-beyerane are Serpukhovian (Fabianska et al., 2003; Izart et al., 583
2006) and Middle Devonian (Sheng et al. 1992; Kashirtsev et al., 2010), 584
respectively. The occurrence of these compounds in Carboniferous 585
sediments that predated the evolution of conifers has been related to 586
the Voltziales (e.g. Schulze and Michaelis, 1990) and/or the close 587
relatives Cordaites (e.g. Disnar and Harouna, 1994 and references 588
therein), while in Devonian coals, these compounds were related to 589
pteridophytes (Sheng et al. 1992). 590
The relative abundance of tricyclic and tetracyclic diterpenoids in 591
coals is affected by the palaeobotanical and palaeoenvironmental 592
conditions and consequently, by the available type of vegetation (e.g. 593
Schulze and Michaelis, 1990; Fleck et al., 2001). In Permian coals, the 594
dominance of tetracyclic diterpenoids, and in particular phyllocladane 595
and kaurane isomers has been related to the pteridosperm Glossopteris, 596
while a predominance of tricyclic terpanes (e.g. isopimarane, rimuane, 597
fichtellite) can particularly indicate a Medullosan pteridosperm input 598
(Noble, 1986). A predominance of kaurane has been also observed in 599
Late Carboniferous rocks (Fabianska et al., 2003) whereas 600
Romero-Sarmiento et al., 29 phyllocladane isomers are more predominant in Lower Carboniferous 601 rocks (Viséan; Disnar and Harouna, 1994). 602 Following the observations of Noble (1986), the abundance of 603 tetracyclic phyllocladanes and kauranes compared to the tricyclic 604 abietane in our coals (Fig. 12) would point to a contribution of non- 605 Medullosan pteridosperms. This is consistent with the spore content of 606 these coals (Spinner and Clayton, 1973), and the described megaflora of 607 Glenarbuck and Pettycur (Scott et al., 1984). In addition, the 608 abundance of ent-beyerane compared to ent-kaurane has been linked to 609 a considerable contribution of Cordaites in late Carboniferous coals 610 from France and Germany (e.g. Schulze and Michaelis, 1990; Vliex et 611 al., 1994; Fleck et al., 2001; Auras et al., 2006). According to these 612 previous studies, the low abundance of ent-beyerane in the Scottish 613 coals can therefore indicate the absence of Cordaites input. This agrees 614 with an absence of Cordaites contribution in the palynological record 615 (Spinner, 1969; Spinner and Clayton, 1973; Scott et al., 1984), and the 616 absence of the arborane/fernane derivatives MATH, MAPH DAPH1 and 617 DAPH2 which have been recently suggested to be of Cordaites origin 618 (Auras et al., 2006). 619 Similarly, abietane, retene, tetrahydroretene, bisnorsimonellite and 620 simonellite (Figs. 13 and 20) have been widely accepted as conifer 621 biomarkers (van Aarssen et al., 2000; Hautevelle et al., 2006). Actually, 622 abietic acid, the major constituent of conifer resin, has been often 623 considered as the biological precursor for retene and its related 624
Romero-Sarmiento et al., 30 aromatic compounds (van Aarssen et al., 2000; Hautevelle et al., 2006). 625 However, most of these compounds, except abietane and 626 bisnorsimonellite, have been recently identified in upper Silurian to 627 lower Devonian sediments and are supposedly associated with early 628 Palaeozoic bryophytes (Romero-Sarmiento et al., 2010). Following Ellis 629 et al. (1996), the higher abundance of alkylphenanthrene compounds 630 compared to isohexylalkylnaphthalenes in the Scottish coals (Fig 13 631 and 19) could indicate a resin acid source. The presence of resinite 632 macerals in both analysed samples (Fig. 4), though in low abundance, 633 gives support to this interpretation. The hypothesis that abietic acid 634 synthesis had already evolved in early land plants, prior to the 635 emergence of true conifers (Romero Sarmiento et al., 2010) gains 636 therefore support with this observation. Similarly, Bray and Anderson 637 (2009) concluded that biosynthetic mechanisms specific to angiosperms 638 had already appeared in the Late Carboniferous, far before the 639 emergence of true angiosperms. 2-Methylretene is commonly associated 640 with retene; it is only known from Permian and younger sediments and 641 considered a conifer biomarker (Bastow et al., 2001). The diaromatic 642 tricyclic totarane and sempervirane also typically co-occur with retene 643 and related hydrocarbons. These biomarkers typically derive from a 644 restricted number of conifer families: mostly Podocarpaceae and 645 Cupressaceae (Otto and Wilde, 2001; Cox et al., 2007), though the 646 totarane skeleton has been also observed in some angiosperms (e.g. 647 Romero-Sarmiento et al., 31 Pinto et al, 1995; Clarkson et al., 2003) and a bryophyte (e.g. Wu and 648 Jong, 2001). 649 In summary, most of the aliphatic and aromatic terrestrial 650 biomarkers observed in the Scottish coals have been related to conifer 651 families (Table 6). The oldest report of conifer megafossils is in the Late 652 Carboniferous (Westphalian B/Late Bashkirian; Scott, 1974) and the 653 oldest occurrence of conifer-related saccate pollens is Early Bashkirian 654 (Zhou, 1994), so that our coals predate the evolution of conifers. Other 655 precursor(s) must therefore be proposed to explain the presence of 656 “conifer biomarkers” in the Viséan Scottish coals. Cordaites are closely 657 related to conifers and have been previously proposed as a source of 658 supposedly typical conifer biomarkers (e.g. Disnar and Harouna, 1994). 659 However, no data support the presence of Cordaites in the flora that 660 contributed to our Scottish coals. An alternative origin for these 661 compounds therefore must be found within the Viséan Scottish flora 662 which was dominated by arborescent or sub-arborescent lycopsids 663 (Lepidodendrales and Isoetales) with a contribution from ferns, 664 sphenopsids and lyginopteridacean pteridosperms. Most of these groups 665 of plants are extinct (Taylor et al., 2009), so that direct comparison with 666 present-day plants is not possible. Pteridosperms are the closest 667 relatives to conifers identified in the Scottish Viséan coals (Taylor et al., 668 2009), and therefore could have provided the “conifer biomarkers”. 669 From the palynological content, pteridosperms were not dominant in 670 the flora of the studied coals (Spinner, 1969; Spinner and Clayton, 671
Romero-Sarmiento et al., 32 1973); a palynological bias is possible, so that pteridosperm biomass 672 would be underestimated in the palynological content (e.g. Phillips et 673 al., 1985; Dimitrova et al., 2005). Pteridosperms, in particular, 674 produced resin (Taylor et al., 2009) and could be at the origin of the 675 resinous secretions observed in our coal samples (Fig. 4C). 676 Lyginopteridacean pteridosperms probably appeared in the latest 677 Devonian (Taylor et al., 2009) but were preceded in the Middle Devonian 678 by progymnosperms, which could therefore be considered for the origin 679 of the “conifer biomarkers” observed in middle Devonian coals (Sheng et 680 al. 1992; Kashirtsev et al., 2010). Progymnosperms, however, were 681 neither described in Chinese nor in Russian Middle Devonian coals 682 (Volkova, 1994; Yi et al., 2007). An alternative source for the “conifer 683 biomarkers” are the arborescent lycopsids (Lepidodendrales) since 684 palynologically, these plants dominated the Scottish flora (Spinner, 685 1969; Spinner and Clayton, 1973). Lepidodendrales already existed 686 during the Devonian (Taylor et al., 2009) and a recent palaeobotanical 687 study showed that the Middle Devonian Chinese coals were dominated 688 by Lepidodendrales and Protolepidodendrales (Yi et al., 2007). From 689 these observations, arborescent lycopsids appear as another likely 690 source for the so-called typical conifer biomarkers. 691
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